1. Field of the Invention
This invention concerns a double conversion tuner and in particular an arrangement whereby both local oscillators of the tuner are substantially stabilized with respect to frequency drift.
2. Description of Related Art
Due to the ever increasing popularity of cable television systems, television receiver manufacturers have had to provide tuners capable of tuning cable as well as broadcast channels. The increased number of channels has increased the possibility for the generation of interference signals by the tuner itself due to cross and intermodulation. While single conversion tuners, i.e., tuners which directly heterodyne (or "mix") selected RF signals to the standard IF range (approximately between 41 and 46 MHz in the United States), can be made to tune both cable as well as broadcast channels with acceptable interference levels, the amount of circuitry required to do so is considerably increased with respect to that required for broadcast channels only. Accordingly, double conversion tuners, i.e., tuners which first heterodyne selected RF signals to produce a first IF signal and then heterodyne the first IF signal to produce a second IF signal in the conventional IF frequency range, have been found to be less expensive than single conversion tuners for tuning cable as well as broadcast channels with acceptable interference levels. This is so since the first IF range can be selected with relative freedom to minimize the generation of interference signals.
In a conventional double conversion tuner there is a first local oscillator for generating a first local oscillator signal with a controllable frequency set in accordance with the selected channel so that the first heterodyning operation causes the first IF signal to be within the preselected first IF frequency range and a second local oscillator for generating a second local oscillator signal with a fixed frequency set so that the second heterodyning operation causes the second IF signal to be within the conventional IF frequency range. The first local oscillator may be a voltage controlled oscillator (VCO) employing a voltage variable capacitance ("varactor") diode which is included in a phase locked loop (PLL) for generating the control voltage for the VCO in accordance with the selected channel. Specifically, the PLL causes the VCO to have a frequency proportionally related to the frequency of a reference frequency signal derived from the output signal of a crystal oscillator by a number determined by the channel number of the selected channel. Because crystal oscillators are extremely stable and accurate, the frequency of the first local oscillator signal is also extremely stable and accurate. While the second local oscillator can also be made stable and accurate by making it a crystal oscillator or including it in a PLL, this is expensive. Accordingly, the second local oscillator is typically made a simple L-C oscillator and is therefore subject to the instability (or "drift") of such oscillators. The drift of the second local oscillator may cause the frequencies of the carriers (i.e., in a television receiver, the picture, color and sound carriers) of the second or conventional IF signal to be offset from their correct or nominal values to the degree that the reproduced image or audio responses may be distorted.
Prior to the advent of phase locked loop tuning control systems, in conventional single conversion tuners, an automatic fine tuning (AFT) signal representing the frequency deviation of the picture carrier of the IF signal was coupled to the local oscillator to correct for its frequency drift. Theoretically, a PLL tuning control system makes the use of an AFT signal unnecessary. However, even where a PLL controls the local oscillator, an AFT signal is often used to control the local oscillator frequency in conjunction with the PLL. This is so because the AFT signal makes it possible to tune RF signals provided by a cable system or a television accessory which, by its operation, may shift or offset the frequencies of RF signals from their nominal or correct values by unpredictable amounts. In essence, the AFT signal is used to offset the local oscillator frequency from the nominal value for the selected channel by an amount necessary to compensate for the frequency offset of the corresponding RF signal.
If the same type of AFT local oscillator control arrangement used in a single conversion tuner is adopted for controlling the first local oscillator of a double conversion tuner in order to correct for RF frequency offsets, the frequency drift of the second local oscillator will, at least in part, also be corrected. This is so because the frequency drift of the second local oscillator is manifested in the second or conventional IF signal (from which the AFT signal is derived) and is counteracted by appropriate changes of the frequency of the first local oscillator signal in response to the AFT signal. However, the frequency drift of the second local oscillator may be so large that the AFT signal cannot control the first local oscillator sufficiently to correct for the accumulated effect of the RF frequency offset and the frequency drift of the second local oscillator. Moreover, this problem is compounded by the fact that the effectiveness of the AFT signal to control the first local oscillator (or the "AFT sensitivity") changes from channel to channel and from tuning band to tuning band. This is so because the control voltage versus channel (or frequency) characteristic of a voltage controlled oscillator is not linear and, in general, a greater change in AFT voltage is required at higher channels than at lower channels to produce a given frequency change. In addition, since the frequency of the first local oscillator signal is changed to compensate for the frequency drift of the second local oscillator, the frequency of the first IF signal will also be changed. If the bandwidth of the first IF section has a narrow passband, e.g., to reduce noise or when a surface acoustic wave device is employed, frequency changes of the first local oscillator signal to compensate for the frequency drift of the second local oscillator may bring the first IF signal outside the passband of the first IF section.